Lithographic plate precursor

Abstract

There is described a method of preparing a lithographic plate which comprises coating on a lithographic support having a hydrophilic surface, a layer of a heat sensitive coating, digitally imaging the coating, then processing the plate with water to remove the unexposed areas of the coating to reveal the hydrophilic surface of the plate and to leave an ink receptive image, wherein the heat sensitive coating comprises a diazo salt of formula (I): wherein R 1 is an anion, R 2 and R 3 represent optional substitution, R 4 is —N— or —S— and R 5 is a group which, after exposure of the plate, renders the residue of the diazo salt oleophilic and fount solution insoluble.

What is claimed is:  
     
       1. A method of preparing a lithographic printing plate using a precursor which comprises on a lithographic support having a hydrophilic surface a layer of a heat sensitive coating, imaging the coating by direct heat and then processing the plate, wherein the plate is processible with pure water to remove the unexposed areas of the coating, to reveal the hydrophilic surface of the plate and to leave an ink receptive image, and wherein the heat sensitive coating comprises a diazo salt of the following formula:                    
       wherein R 1  is an anion, R 2  and R 3  represent optional substitution, R 4  is —N— or —S— and R 5  is a group which after exposure of the plate renders the residue of the diazo salt oleophilic and fount insoluble. 
     
     
       2. The method according to claim  1  wherein R 1  is sulphate, nitrate, chloride or perfluorooctyl. 
     
     
       3. The method according to claim  1  wherein R 2  and R 3  when present are selected from the group consisting of alkoxy, halogen, alkyl, and amine. 
     
     
       4. The method according to claim  1  wherein when R 4  is —N— the moiety —R 4 —R 5  is of the formula:                    
       where R 7  and R 6  are part of a polymer chain or are part of a morpholino functional group.  
     
     
       5. The method according to claim  1  wherein when R 4  is —N— the moiety —R 4 —R 5  is of the formula:                    
       where R 6  is hydrogen and R 7  is a group of the formula:                    
       where R 8  is the residue of a condensing agent and n is 1 to 100.  
     
     
       6. The method according to claim  1  wherein the direct heat source is controlled by an image stored in a computer. 
     
     
       7. The method according to claim  1  wherein the heat sensitive coating further comprises a heat absorbing compound. 
     
     
       8. The method according to claim  7  wherein the heat absorbing compound is a pigment. 
     
     
       9. The method according to claim  8  wherein the pigment is selected from the group consisting of carbon black, nigrasine, green and graphite. 
     
     
       10. The method according to claim  8  wherein the pigment is an organic pigment. 
     
     
       11. The method according to claim  7  wherein the heat absorbing compound is a dye selected from the group consisting of the squarylium, merocyanine, indolizine, pyrylium, and metal dithioline classes. 
     
     
       12. The method according to claim  1  wherein the lithographic support is Al, Zn, polyester film or a paper base. 
     
     
       13. The method according to claim  12  further comprising treating or coating the surface of the support to render the support hydrophilic.